1. Field of the Invention
This invention relates to a chuck for holding a semiconductor wafer shaped device during operation of a lithography machine, and more particularly, to such a chuck having a size as large as the wafer shaped device and further having a mechanism to permit the wafer devices to be placed on such chuck by only contacting the bottom of the wafer shaped device.
2. Description of the Prior Art
Semiconductor devices have for many years been fabricated in part by using a lithography machine for creating certain patterns on a photoresist covered semiconductor wafer. Until recently, visible or ultraviolet light has been used as the energy source to expose the resist covered wafer. However, recent advances in semiconductor processing techniques have required features smaller than can be easily patterned by conventional lithography machines. In order to pattern smaller features on a semiconductor wafer, new, shorter wavelength, energy sources are required. The next logical energy source is the X ray.
One example of X-ray lithography machines is shown in U.S. Pat. No. 4,870,668 in the name of Frankel et al and entitled "Gap Sensing/Adjustment Apparatus And Method For A Lithography Machine". In the type of machine shown in the Frankel et al patent, a source of X-rays is emitted from a plasma created by focusing a laser beam on a metal target. The X-rays travel through an exposure column towards a mask mounted at the end thereof. The resist covered wafer is moved in discrete steps across the mask, with a very small gap existing between the mask and the wafer surface being exposed. In order to take advantage of the small features which can be patterned using the X-ray lithography machine of the Frankel et al patent, a very precise and repeatable translation device, or stepper mechanism, is required and is described therein. Such a device includes a wafer chuck for holding the wafer being exposed as well as various translating means for moving the chuck in extremely accurate and minute steps.
In order to accurately and precisely use the X-ray lithography machine of the Frankel et al patent, it is necessary to precisely align each section of the wafer being exposed relative to the mask. This, in turn, requires that the mask be properly attached to the exposure column and the wafer be placed on the stepper mechanism. In using the stepper mechanism, care must be taken to avoid damaging the wafers and mask, particularly when placing these items on and off the wafer chuck portion of the stepper mechanism. This is particularly true with respect to the mask, which is an extremely delicate item.
Because of the precision required in an X-ray lithography mechanism of the type described by the Frankel et al U.S. Pat. No. 4,870,668, it is necessary to individually level each section of the wafer relative to the mask because of imperfections found on the flatness of the wafers. In view of such precision requirements, it is necessary to hold the wafers very carefully, preferably in a vacuum chuck having an evacuated holding area the same size and shape as the wafer itself. Such a chuck is shown in U.S. Pat. No. 4,213,698 in the name of Firtion et al entitled, "Apparatus and Method For Holding And Planarizing Thin Workpieces".
In the past, wafers have generally been placed on wafer chucks by robots. However, automatic or robotic operation required a relatively small chuck plate upon which the wafer rested in order to allow the robot arm to grasp the wafer from beneath. Alternatively, wafers have been held from the sides or top, which is undesirable because of the damage to the partially processed wafer, or wasteful of wafer area if special gripping areas are provided to hold the wafer. In some instances, the wafer itself is slid onto the wafer chuck plate which makes alignment very difficult. In the above noted Firtion et al U.S. Pat. No. 4,213,698, a pair of tweezers was used to place the wafer on the chuck plate. What is needed is a more accurate and less damaging technique and apparatus to load and unload the wafers from the wafer chuck.
In the past, masks have been installed in the lithography machine individually by hand, or by special mask handling apparatus. Because of the critical relationship between an X-ray mask and the wafer being exposed, as well as the delicate nature of X-ray mask, extreme care must me taken to properly position and affix the X-ray mask. Mask replacements or changes invariably result in significant delays in the processing of wafers, particularly where masks must be changed often. Mask changing also results in additional apparatus having to be provided if the mask installation is to be mechanized. What is needed is a simple means, utilizing the existing wafer handling apparatus used for positioning the wafer during the exposure processing, which can additionally be used to install the masks.
An example of wafer handling and mask handling apparatus is shown in U.S. Pat. No. 4,516,253 to W. Thomas Novak, entitled "Lithography System". An example of a mask tray used in loading a mask onto an X-ray lithography machine is shown in U.S. Pat. No. 4,549,843 to Peter R, Jagusch et al entitled "Mask Loading Apparatus, Method and Cassette".